Burstein-Moss effect
The Burstein-Moss effect is a quantum mechanical effect that occurs in degenerate semiconductors . He describes how the effective band gap increases in semiconductors with a very high level of doping , which in optical spectroscopy is expressed in a shift in the absorption edge as a function of the charge carrier density, hence also referred to as the Burstein-Moss shift . The effect was originally described on indium antimonide (InSb) by Elias Burstein and Trevor S. Moss independently of one another.
Explanation
In the case of undoped semiconductors, the valence band fully occupied with electrons and the unoccupied conduction band (at absolute zero ) are energetically separated by the band gap. The introduction of foreign atoms creates localized energy levels within this band gap that do not interact with one another and, as an intermediate level, increase the electrical conductivity, cf. Defect line . The Burstein-Moss effect is based on the interaction of donor levels , that is, impurity levels that are above the Fermi level near the conduction band edge. Whether donor or acceptor levels develop when a certain foreign element is introduced depends on the semiconductor material under consideration.
As mentioned, only localized donor levels are initially formed at low doping concentrations. As the doping concentration increases, the number of donor levels increases. If it exceeds a critical density, the donor levels also interact with one another and form energy bands (impurity bands) within the original band gap. If the concentration is exceeded, the donor bands merge with the conduction band and electrons can now switch from the donor level to the conduction band without bridging energy jumps. The Fermi energy is no longer within the band gap, but in the area of the conduction band. The semiconductor is called degenerate and now has a similarly high electrical conductivity as metals.
Since the Fermi level is shifted to energies greater than the original conduction band edge and all energy levels below the Fermi level are occupied by electrons, higher energies have to be expended in order to excite electrons from the valence band into the conduction band. In absorption spectroscopy , this is shown by a shift in the absorption edge towards higher energies (band gap energy + Burstein-Moss shift), i.e. towards shorter wavelengths.
Another effect opposes the band gap widening effect. At high charge carrier concentrations, many-particle effects occur, i.e. effects that only come about through the interactions of several particles. Electron-electron and electron-donor interactions lead to changes in the band structure , the so-called band renormalization . This leads to a lowering of the conduction band edge and an increase in the valence band edge. This bending reduces the band gap energy and can lead to the fact that the Burstein-Moss effect cannot be observed.
Web links
- Christian Koepf: 5.1.3 Dependency on funding. In: Modeling the electron transport in compound semiconductor alloys. November 11, 1997, accessed July 24, 2014 (dissertation).
Individual evidence
- ↑ Elias Burstein: Anomalous Optical Absorption Limit in InSb . In: Physical Review . tape 93 , no. 3 , February 1954, p. 632-633 , doi : 10.1103 / PhysRev.93.632 .
- ^ TS Moss: The Interpretation of the Properties of Indium Antimonide . In: Proceedings of the Physical Society. Section B . tape 67 , no. October 10 , 1954, p. 775 , doi : 10.1088 / 0370-1301 / 67/10/306 .